1. Field of the Invention
The present invention relates to a tire pneumatic pressure detector. More specifically, the present invention relates to a tire pneumatic pressure detector used for generating warnings or the like after detecting a decline in the tire pneumatic pressure.
2. Description of Related Art
Using the fact that tire radius changes according to pneumatic pressure, one known conventional method of detecting tire pneumatic pressure in real-time predicts tire pneumatic pressure by detecting different vehicle wheel speeds at the same vehicle speed. However, there are great differences in tire radii due to wear and tear, and tire radius is readily affected by such vehicle cruising states like turning or braking. Also, with radial tires or the like which have been in remarkably widespread use in recent years, the tire radius changes only by a minute amount, e.g., 1 mm, even if the pneumatic pressure decreases by 1 kg/cm.sup.2, and thus, for this reason accurate prediction of the pneumatic pressure from the change in the tire radius is difficult.
Accordingly, focusing on the fact that spring vibration frequency components include tire resonance frequency (caused by tire torsion vibration and coupled vibration of the front and back springs of suspensions), the inventors have previously proposed an apparatus as disclosed in Japanese Laid-Open Patent Publication No. Hei 6-238920 which predicts tire pneumatic pressure from the resonance frequency or spring constant.
While the above proposed apparatus predicts tire pneumatic pressure more accurately than the conventional method based on tire radius, this method can be further improved.
There is room for improvement because the resonance frequency of the suspension system changes if the tires or wheels are replaced even for the same tire pneumatic pressure, and as a result, errors occur in the detection of pneumatic pressure and false warnings of abnormal pneumatic pressure are generated.
It must be noted here that abnormal pneumatic pressure refers to that state of tire pneumatic pressure that affects tire and vehicular functions such as contact of the tire with the ground, tire life, fuel consumption and the like.